1. Field of Invention
The present invention relates to a light guide plate emitting light from an emission face after applying direction-conversion within the light guide plate to light applied thereto sideways, and to a surface light source device and display employing the light guide plate.
2. Related Art
A well-known technique gives a surface light source device that employs a light guide plate having an incidence end face provided by a side end face and an emission face provided by one of two major faces (i.e. faces larger than end faces), wherein light is introduced into the light guide plate through incidence end face, the surface light source device being applied to various uses such as back-lighting for a liquid crystal display. A surface light source device of such a type has a basic performance greatly depending on a light guide plates employed therein.
A basic function of a light guide plate is to change a propagation direction (roughly in parallel with an emission face of the light guide plate) of light introduced into the light guide plate through a side end face so that the light is emitted through the emission face. As known well, a simply transparent light guide plate without any modification is capable of deflecting light a little, providing a unsatisfactory brightness. Therefore any means for promoting emission through the emission face is required.
The present invention proposed a light guide plate provided with so-called micro-reflectors and surface light source device/LCD employing the light guide plate, being disclosed in PCT/JP00-00871 (WO00-49432).
According to the this prior art, a light guide plate has a back face provided with a great number of micro-reflectors. FIG. 6 shows one of the micro-reflectors to illustrate an emission promotion function. As shown in FIG. 6, each micro-reflector has a first and second reflection surfaces which are a pair of inclined reflection surfaces to provide a valley inside. Upon reaching, a micro-reflector and entering into a valley thereof on the way of propagation within the light guide plate, light is direction-converted mainly by inner-reflections effected twice. This twice-reflected inner reflection consists of two inner reflections, one inner reflection being effected at one of the first and second reflection surfaces and the other inner reflection then being effected at the other of the first and second reflection surfaces.
This direction-conversion produces light directed to the emission face, promoting emission from the emission face. A propagation direction of light after being direction-converted by each micro-reflector mainly depends on the inclinations (i.e. spatial directions) of the first and second reflection surfaces of the micro-reflector. Therefore angular emission characteristics of the light guide plate as a whole can be controlled within a remarkable range ny means of adjusting factors such as direction distribution of reflection surfaces, or arraying pattern or distribution of the micro-reflectors.
According to typical designs, it is aimed that a light guide plate allows an emission face of the light guide plate to emit the most intensive light toward a generally frontal direction Employable primary light sources fro supplying light to the light guide plate include a line-like light source such as cold cathode lamp and point-like light source such as light emitting diode, both being usable for providing a surface light source device to have an improved brightness.
It is, however, not rare for a light guide plate or surface light source device provided with micro-reflectors as above-mentioned to be wanted to have further improved characteristics. For example, the followings are demanded.
(i) A micro-reflector intensively helps an emission to emit light toward a generally frontal direction However, some light is incident to a micro-reflector at a large inclination angle with respect to a ridge formed by a meeting of two slopes for effecting direction-conversion, hardly resulting in a promoted frontal emission because such obliquely incident light is converted into beams which is rather divergent regarding in a plane parallel to an incidence end face. Emission characteristics are wanted in some cases to be corrected for avoiding such situation.
(ii) Indeed a high emission directivity is obtained by relying upon direction conversion effected by micro-reflectors, but such a high emission directivity is undesirable in some cases because of being excessive. In other words, an unnaturally reduced brightness can be felt when a direction of viewing is slightly deviated from a particular direction (frontal direction, typically). Emission characteristics are wanted to be corrected in some cases for avoiding this.
(iii) A direction conversion effect relying upon micro-reflectors is apt to bring a fine brightness unevenness which appears depending on positions of the micro-reflector (presence/blank). Such a brightness unevenness gives a feeling of glaring. Emission characteristics are wanted to be corrected in some cases for avoiding this.
The the present invention is proposed under the above-described background, aiming to provide an art that enables a light guide plate, surface light source device and display, which utilize micro reflectors, to have further adjusted characteristics.
In the first place, the present invention is applied to a light guide plate which has an emission face provided by a major face, a back face opposite with the emission face and an incidence face for introducing light.
According to a feature of the present invention, the back is provided with a great number of projection-like micro-reflectors and a fine-unevenness surface formed among the micro-reflectors. Each of the micro-reflectors has at least a pair of slopes which meet each other as to form a ridge that gets closer with an increasing distance from the incidence face.
This feature enables a light guide plate to output an illumination light with characteristics which include a good directivity indebted to the micro-reflectors and is further adjusted by the fine-unevenness surface. In a typical case, the micro-reflectors are shaped like quadrangle pyramids.
In the next place, the present invention is applied to a surface light source device which has a light guide plate having an emission face provided by a major face, a back face opposite with the emission face and an incidence face for introducing light, and a primary light source for supply light to the incidence face. The light guide plate is featured as above.
That is, the back is provided with a great number of projection-like micro-reflectors and a fine-unevenness surface formed among the micro-reflectors. Each of the micro-reflectors has at least a pair of slopes which meet each other as to form a ridge that gets closer with an increasing distance from the incidence face. In a typical case, the micro-reflectors are shaped like quadrangle pyramids.
The merits of the light guide plate are maintained in the surface light source device. In other words. Obtained is a surface light source device which is able to output an illumination light with characteristics which include a good directivity indebted to the micro-reflectors and is further adjusted by the fine-unevenness surface. A particular need for a member for direction-modifying the emission (for example, prism sheet) is cancelled.
The present invention is also applied to a display comprising a liquid crystal display panel and a surface light source device for illumination the liquid crystal display panel. The surface light source device is featured as above.
The merits of the surface light source device are maintained in the display. In other words, Obtained is a display which allows to look bright as viewed from a generally frontal direction because the liquid crystal display panel is effectively supplied with an illumination light with characteristics which include a good directivity indebted to the micro-reflectors and is further adjusted by the fine-unevenness surface.